Focus control method and focus controller

ABSTRACT

The present invention aims to perform high-accuracy focus control in an optical disk drive, even if a solid immersion lens (SIL) or a solid immersion mirror (SIM) is set between an objective lens and the optical disk in order to increase a numerical aperture, by accurately detecting a change of the distance between an SIL or SIM and an optical disk. 
     The present invention comprises light detection means ( 8 ) for detecting the light incoming to an SIL ( 2 ) or an SIM from an objective lens ( 1 ) at an incidence angle at which a numerical aperture is larger than 1 and reflected from a plane of the SIL ( 2 ) or SIM facing an optical disk, light detection means  10  for detecting the light incoming to the objective lens ( 1 ), comparison means ( 11 ) for finding a ratio between light amounts of the light detected by light detection means ( 8 ) and ( 10 ), and control means ( 11 ) for controlling operations of a focus actuator in accordance with the ratio.

TECHNICAL FIELD

The present invention relates to, for example, a distance-changedetecting method for detecting a change of the distance between anoptical system and an object to be irradiated (e.g. optical recordingmedium) and controlling the distance, a distance-change detector, afocus control method, a focus controller, and totally-reflected-lightdetecting method, particularly to a method and an apparatus to bepreferably applied to an optical system provided with optical means forincreasing a numerical aperture such as a solid immersion lens or solidimmersion mirror.

BACKGROUND ART

Information is recorded or reproduced in or from an optical disk bynarrowing down a laser beam to a very-small spot diameter and making thelaser beam irradiate a recording surface of an optical recording medium.

To make the laser beam having a certain spot diameter irradiate arecording surface, it is necessary that a distance between an objectivelens for condensing the laser beam and an optical disk is kept within arange in which a shift between a focal position of the objective lensand the recording surface falls within a focal depth of the objectivelens.

For this reason, an optical disk drive for reproducing data from anoptical disk (or recording and reproducing data in and from a rewritableoptical disk) or an exposure device for a master optical disk detects achange of the distance between an objective lens and an optical disk,moves the objective lens in the optical-axis direction of a laser beamin accordance with the detection result, and thereby performs control ofthe distance (focus control).

As the focus control method, for example, the off-axis method,astigmatism method, and knife-edge method have been used so far. All ofthese methods detect a change of the distance between an objective lensand an optical disk using the light reflected from a signal-recordingsurface of an optical disk.

To make the spot diameter still smaller in response to needs ofhigh-density optical disk, a numerical aperture larger than that of anobjective lens itself (e.g. numerical aperture larger than 1) hasrecently been realized by intervening an solid immersion lens (SIL) of ashape obtained by cutting off a part of a spherical lens and having ahigh refractive index between an objective lens and an optical diskfacing the spherical surface of the solid immersion lens and the planeopposite to the spherical surface toward the objective lens and opticaldisk, respectively. Also, it is possible to realize a large numericalaperture by a solid immersion mirror (SIM).

When a numerical aperture is larger than 1, an intensity of a laser beamto irradiate an optical disk extremely lowers if the distance between anSIL and the optical disk exceeds a near field (a range of almost thewavelength of the laser beam). Therefore, it is necessary to performfocus control so that the distance may be kept constant within the rangeof the near field.

However, in a very small range such as the near field, even if thedistance between the SIL and the optical disk changes, a change of thelight reflected from the optical disk is very small. Thus, it isdifficult to accurately detect a change of the distance between anobjective lens and the optical disk in accordance with the change of thereflected light.

Therefore, when setting an SIL between an objective lens and an opticaldisk, it is difficult to perform high-accuracy focus control by theconventional focus control method.

The same problem occurs in a beam irradiator such as an exposure deviceused for manufacturing a master optical disk for obtaining aninformation-recording medium such as an optical disk.

It is an object of the present invention to provide a distance-changedetecting method, a distance-change detector, a focus control method anda focus controller, as well as a totally-reflected-light detectingmethod which enables to perform high-accuracy focus control byaccurately detecting a change of the distance between an SIL or SIM andan optical disk even if an optical means for increasing a numericalaperture such as the SIL or SIM is employed.

DISCLOSURE OF THE INVENTION

The present invention proposes a distance-change detecting method fordetecting a change of the distance between a second optical means of anoptical system having a first optical means for condensing the light toirradiate an optical recording medium and the second optical means setbetween the first optical means and the optical recording medium inorder to realize a numerical aperture larger than the numerical apertureof the first optical means, wherein the light entering the secondoptical means from the first optical means at an incidence angle atwhich the numerical aperture is larger than a predetermined value andreflected from a plane of the second optical means facing the opticalrecording medium is detected and a distance change is detected inaccordance with the light amount of the reflected light.

This distance-change detecting method makes it possible to detect thelight entering the second optical means from the first optical means atan incidence angle at which a numerical aperture is larger than 1 andreflected from a plane of the second optical means facing the opticalrecording medium using a solid immersion lens or solid immersion mirroras the second optical means.

The present invention proposes a distance-change detector for detectinga change of the distance between a second optical means and an opticalrecording medium of an optical system having a first optical means forcondensing the light to irradiate the optical recording medium and thesecond optical means set between the first optical means and the opticalrecording medium in order to realize a numerical aperture larger thanthat of the first optical means, the detector comprising a lightdetection means for detecting the light entering the second opticalmeans from the first optical means at an incidence angle at which thenumerical aperture is larger than a predetermined value and reflectedfrom a plane of the second optical means facing the optical recordingmedium and a detection means for detecting the distance change inaccordance with the luminous energy of the reflected light detected bythe light detection means.

This distance-change detector makes it possible to arrange that thelight detection means can detect the light entering the second opticalmeans from the first optical means at an incidence angle at which anumerical aperture is larger than 1 and reflected from a plane of thesecond optical means facing the optical recording medium using a solidimmersion lens or a solid immersion mirror as the second optical means.

The present invention proposes a focus control method for controllingthe distance between a second optical means and an optical recordingmedium of an optical system having a first optical means for condensingthe light to irradiate the optical recording medium and the secondoptical means set between the first optical means and the opticalrecording medium in order to realize a numerical aperture larger thanthat of the first optical means, wherein the light entering the secondoptical means from the first optical means at an incidence angle atwhich the numerical aperture is larger than a predetermined value andreflected from a plane of the second optical means facing the opticalrecording medium is detected, a distance change being detected inaccordance with the luminous energy of the reflected light, and thedistance being controlled in accordance with the detection result.

The present invention proposes a focus control method of a convergentlens for floating a convergent lens above the surface of an irradiatedobject by positive-pressure air and attracting the convergent lenstoward the irradiated object by negative-pressure air.

This focus control method controls the distance between the convergentlens and the irradiated object by controlling the air pressure.

Moreover, the focus control method using the air pressure makes itpossible to drive the convergent lens by an electrical driving means andcorrect a focal position.

The present invention proposes a focus controller for controlling thedistance between a second optical means and an optical recording mediumof an optical system having a first optical means for condensing thelight to irradiate an optical recording medium and the second opticalmeans set between the first optical means and the optical recordingmedium in order to realize a numerical aperture larger than that of thefirst optical means, the controller comprising a light detection meansfor detecting the light entering the second optical means from the firstoptical means at an incidence angle at which the numerical aperture islarger than a predetermined value and reflected from a plane of thesecond optical means facing the optical recording medium, a detectionmeans for detecting the distance change in accordance with the luminousenergy of the reflected light detected by the light detection means, anda control means for controlling operations of a focus actuator inaccordance with a detection result of the detection means.

Moreover, the present invention proposes a totally-reflected-lightdetecting method for detecting the light totally reflected from aconvergent lens by making a laser beam pass through a polarized-beamsplitter and a ¼-wavelength plate and enter the convergent lensincluding a solid immersion lens or solid immersion mirror, and makingthe light returned from the convergent lens enter the polarized-beamsplitter again to separate the light in a direction different from thatof an incoming-beam source.

Furthermore, the present invention proposes a totally-reflected-lightdetecting method for detecting the light totally reflected from thebottom plane of a convergent lens including a solid immersion lens orsolid immersion mirror by arranging an opaque mask on an optical path ofthe light returned from the convergent lens and interrupting theinterference light of the returned light interfering on a plurality ofplanes including an end face of the convergent lens.

These inventions are proposed for the following reasons.

As already described on an objective lens and an SIL by way of example,when a numerical aperture realized by a first optical means(corresponding to an objective lens) and second optical means(corresponding to an SIL) becomes larger than a predetermined value, theintensity of a laser beam to irradiate an optical recording mediumextremely lowers if the distance between the second optical means andthe optical recording medium exceeds the near field.

This is because, when the SIL contacts with the optical recordingmedium, almost all the light (hereinafter also referred to as“high-frequency component” of incoming light) entering an SIL from anobjective lens at an incidence angle at which a numerical aperture islarger than a predetermined value permeates a plane of the SIL facing anoptical recording medium (hereinafter also referred to as “facingplane”) and irradiates the optical recording medium. However, areflectance of the high-frequency component at the facing plane suddenlyincreases as the SIL moves away from the optical recording medium andalmost 100% of the high-frequency component is reflected from the facingplane when the SIL separates from the optical recording medium exceedingthe near field.

In this manner, even in a very small range such as the near field, achange of the light amount of the reflected light of the high-frequencycomponent at the facing plane is sufficiently large.

Therefore, by detecting the reflected light, it is possible toaccurately detect a change of the distance between a second opticalmeans and an optical recording medium in accordance with thelight-amount change and perform accurate focus control.

The light amount of the reflected light changes proportionally to achange of the intensity of the light incoming to the second opticalmeans (i,e, a change of the intensity of the light emitted from a lightsource and coming into the first optical means) even if the distancebetween the second optical means and the optical recording medium isconstant.

Therefore, in these distance-change detecting method and distance-changedetector, for example, it is more preferable to detect not only thereflected light but also the light incoming to either of the firstoptical means and the second optical means and find a ratio betweenlight amounts of the reflected light and the incoming light. Thedistance-change detector further comprises a second light detectionmeans for detecting either of the light incoming to the first opticalmeans and the light incoming to the second optical means and thedetection means for detecting the above distance change is comprised ofa comparison means for finding the ratio between light amounts of thereflected light detected by the light detection means and the incominglight detected by the second light-detection means.

Even if the intensity of incoming light changes, the above ratio doesnot change because the light amount of the reflected light and that ofthe incoming light change at the same rate. Accordingly, it is possibleto detect a change of the distance between the second optical means andthe optical recording medium regardless of the intensity of the incominglight.

Moreover, the above incoming-light detection can be performed withoutchanging the shape of the facing plane, etc. of the second optical meansfrom a conventional one. Furthermore, in the case of arecording/reproducing apparatus for an optical recording medium or anexposure device for a master disk of the optical recording medium, amonitoring photodetector for controlling an output of a semiconductorlaser forming a light source can directly be used as it is to find theabove ratio. Therefore, it is possible to minimize the number of devicesto be added.

Next, when detecting the reflected light of the high-frequency componentat the facing plane, if the light (component other than thehigh-frequency component) incoming to the second optical means at anincidence angle at which a numerical aperture becomes equal to or lessthan a predetermined value is also reflected from the facing plane orthe light is reflected from a portion of an optical recording mediumcloser to the second optical means than the signal recording plane ofthe optical recording medium, a detection accuracy is deterioratedbecause interference occurs between these lights. As a result, anaccuracy for detecting a change of the distance between the secondoptical means and optical recording medium may be deteriorated.

Therefore, in the distance-change detector, it is more preferable toset, for example, a member for controlling light reflection at least oneither of the facing plane of the second optical means and a portion ofthe optical recording medium closer to the second optical means than thesignal recording plane of the medium.

This causes, the interference between the reflected light of thehigh-frequency component and other light occurring at the facing planeto be decreased. Therefore, deterioration of the detection accuracy of achange of the distance between the second optical means and opticalrecording medium due to the interference will be suppressed.

When controlling the distance between a convergent lens and anirradiated object using positive-pressure air and negative-pressure air,focus control of a lower-frequency band is stabilized.

When converting a linearly-polarized laser beam emitted through apolarized-beam splitter into a circularly-polarized beam by a¼-wavelength plate, making then the beam enter a convergent lens,changing the polarized direction of the light returned from theconvergent lens by the ¼-wavelength plate, and making the returned lightenter the polarized-beam splitter again, it is possible to detecttotally-reflected returned light at larger intensity and further reducenoises by separating most of power of the returned light in a directiondifferent from that of an incoming-beam source.

By arranging an opaque mask on an optical path of the light returnedfrom a convergent lens including a solid immersion lens or solidimmersion mirror, the intensity of interference light forming noises iscontrolled and therefore, it is possible to obtain a large ratio ofreturned light of a high-frequency component, namely, a detection signalto a noise intensity (S/N ratio).

As described above, according to the distance-change detecting methodand distance-change detector of the present invention, an advantage canbe obtained that it is possible to accurately detect a change of thedistance between a second optical means and an optical recording mediumin an optical system having a first optical means for condensing thelight to irradiate the optical recording medium and a second opticalmeans set between the first optical means and the optical recordingmedium in order to realize a numerical aperture larger than that of thefirst optical means, such as an optical system a solid immersion lensset between an objective lens and an optical disk.

Moreover, according to the focus control method and focus controller ofthe present invention, an advantage can be obtained that it is possibleto accurately control the distance between the second optical means andoptical recording medium in the above optical system within a range ofthe near field.

Furthermore, when detecting not only reflected light but also either ofthe light incoming to the first optical means and the light incoming tothe second optical means so as to find the ratio between light amountsof the reflected light and the incoming light, it is possible to detecta change of the distance between the second optical means and opticalrecording medium irrespective of the intensity of incoming light.

Moreover, the above incoming-light detection can be performed withoutchanging the shape of the facing plane of the second optical means froma conventional one. Furthermore, in the case of a recording/reproducingapparatus for an optical recording medium or an exposure device for amaster disk of the optical recording medium, for example, a monitoringphotodetector for controlling an output of a semiconductor laser being alight source can be used as it is to find the above ratio. Thus, it ispossible to minimize the number of devices to be newly added and thelike.

In addition, when setting a member for controlling light reflection atleast on either of the facing plane of the second optical means and aportion of the optical recording medium closer to the second opticalmeans than the signal recording plane of the medium, deterioration of anaccuracy for detecting a change of the distance between the secondoptical means and optical recording medium due to the interferencebetween the reflected light of a high-frequency component and otherlight occurring at the facing plane is suppressed. Therefore, it ispossible to more accurately detect the distance change.

When forming an air film between a convergent lens and an irradiatedobject by positive-pressure air and negative-pressure air and performingfocus control by the air film, it is possible to follow a swell in alower-frequency band of the irradiated object. In other words, it ispossible to stably follow the swell at a constant response rate.

Moreover, by controlling an air pressure, it is possible to performfocus control at a higher response rate.

When driving a convergent lens by an electric driving means in focuscontrol and correcting a focal position, it is possible to follow aswell of the irradiated irradiated object in a high-frequency band andperform accurate focus control.

According to a totally-reflected-light detecting method for detectingthe light totally-reflected from a convergent lens by making a laserbeam pass through a polarized-beam splitter and a ¼-wavelength beamsplitter and enter the convergent lens, making the light returned fromthe convergent lens enter the polarized-beam splitter again, andseparating the returned light in a direction different from that of anincoming-beam source, it is possible to detect totally-reflectedreturned light at the large intensity.

According to a totally-reflected-light detecting method for detectingthe light totally-reflected from the bottom plane of a convergent lensby arranging an opaque mask on an optical path of the light returnedfrom the convergent lens and interrupting the light of the returnedlight interfering on a plurality of planes including an end face of theconvergent lens, it is possible to control the intensity of interferencelight forming noises and detect the totally-reflected returned light ofa high-frequency component.

Therefore, when applying the method to focus control, it is possible toobtain a large ratio of detection-signal intensity to noise intensity(S/N ratio) in focus control, thus making the control to be performedwith higher accuracy.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a side view showing by way of example an optical system of anoptical disk drive to which the present invention is applied and anoptical disk set on the optical disk drive.

FIG. 2 is an illustration showing by way of example a part of an opticalpickup of an optical disk drive to which the present invention isapplied and a focus control system of the optical disk drive.

FIG. 3 is a schematic sectional view of a convergent lens portion of abeam irradiator using the present invention.

FIG. 4 is a top view of an example of essential portion of a focuscontrol mechanism in an apparatus of the present invention.

FIG. 5 is a top view of another example of essential portion of a focuscontrol mechanism in an apparatus of the present invention.

FIG. 6 is a sectional view of an SIM used in a convergent lens of anapparatus of the present invention.

FIG. 7 is a schematic sectional view of another convergent lens portionof a beam irradiator using the present invention.

FIG. 8 is a structural diagram showing a method and an apparatus foradjusting a tilt of a convergent lens of an apparatus of the presentinvention.

FIG. 9 is a structural diagram showing a totally-reflected-lightdetecting method used for focus control of an apparatus of the presentinvention.

FIG. 10 is an illustration of tilt adjustment of a convergent lens.

FIG. 11, is an illustration of totally-reflected-light detection.

FIG. 12 is an illustration of tilt adjustment of a convergent lens.

BEST MODE FOR CARRYING OUT THE INVENTION

In the following, an example of applying the present invention to anoptical disk drive will be described.

FIG. 1 shows by way of example an optical system provided in an opticalpickup of an optical disk drive in order to condense a laser beam to beirradiated to an optical disk and an optical disk set on the opticaldisk drive.

This optical system comprises an objective lens 1 for condensing a laserbeam L and a solid immersion lens (SIL) 2 set between an optical disk 3and the objective lens 1. The objective lens 1 and the SIL 2 areintegrally moved by a focus actuator (not illustrated) in theoptical-axis direction of the laser beam L.

The SIL 2 is a lens formed by cutting off a part of a spherical lens inshape and having a refractive index n, which is set so that itsspherical surface may be faced toward the objective lens 1 and the planeopposite to the spherical surface may be faced toward the optical disk3.

The relation between an numerical aperture NA realized by the objectivelens 1 and SIL 2, a refractive index n of the SIL 2, and a maximumincidence angle θ_(max) of the laser beam L condensed by the objectivelens 1 to the SIL 2 is shown by the following expression as is wellknown.NA=n·sin θ_(max)

In this example, the refractive index n and the maximum incidence angleθ_(max) are set so that a product n·sin θ₀ including a certain incidenceangle θ₀ smaller than the maximum incidence angle θ_(max) and therefractive index n may be equal to 1 as shown in FIG. 1.

As a result, a numerical aperture larger than 1 is realized by theobjective lens 1 and SIL 2. Therefore, when the SIL 2 contacts with theoptical disk 3, almost all the laser beam (high-frequencycomponent)(laser beam at the hatched portion in FIG. 1) incoming to theSIL 2 from the objective lens 1 at an incidence angle larger than θ₀(i,e, an incidence angle at which a numerical aperture becomes largerthan 1) permeates a plane of the SIL 2 facing the optical disk 3 and isirradiated to the optical disk 3. However, as the SIL 2 moves away fromthe optical disk 3, a reflectance of the high-frequency component at thefacing plane suddenly increases. When the SIL 2 moves away from theoptical disk 3 beyond the near field, almost 100% of the high-frequencycomponent is reflected from the facing plane.

An antireflection film 4 is formed on the facing plane of the SIL 2 forcontrolling the reflection of a laser beam incoming to the SIL 2 at anincidence angle of θ₀ or less (component other than high-frequencycomponent) (i,e, an incidence angle at which a numerical aperturebecomes 1 or less.

Also, the antireflection film 4 is formed on a portion of the opticaldisk 3 closer to the SIL 2 than the signal recording plane of the disk 3(e.g. the surface facing the SIL 2).

In this way, the reflection of the laser beam incoming to the SIL 2 atan incidence angle equal to or smaller than θ₀ from the facing plane ofthe SIL 2 and the reflection of the laser beam from a portion of theoptical disk 3 closer to the SIL 2 than the signal recording plane ofthe disk 3 are suppressed.

FIG. 2 shows an example of a portion of the optical pickup relating tothe present invention and a focus control system of the optical diskdrive.

A linearly-polarized laser beam L emitted from a semiconductor laser(not illustrated) is converted into parallel rays by a collimator lens(not illustrated) in the optical pickup, and a polarized plane of theparallel rays is rotated by a ½-wavelength plate (not illustrated)before the parallel rays enter a polarized-beam splitter (PBS) 5.

A part of the incoming beam is reflected from the PBS 5 and detected bya photodetector (PD) 10 for monitoring the intensity of the laser beamthrough a condenser lens 9.

Moreover, most of the beam incoming to the PBS 5 passes through the PBS5 and is converted into circularly-polarized rays by a ¼-wavelengthplate 6 to be condensed by the objective lens 1, and enters the SIL 2.

A photodetector (PD) 8 is provided through a condenser lens 7 on anoptical path where the above laser beam incoming to the SIL 2 from theobjective lens 1 at an incidence angle larger than θ₀ (high-frequencycomponent) (laser beam at the hatched portion in FIG. 2) is reflectedfrom the facing plane of the SIL 2, passes through the objective lens 1,converted into a linearly polarized light orthogonal to the initial oneby the ¼-wavelength plate 6, and reflected from the PBS 5.

Thus, the reflected light of the high-frequency component at the facingplane of the SIL 2 is detected by the PD 8.

A signal showing a light amount p detected by the PD 8 is sent to anarithmetic circuit 11.

A signal showing a light amount q detected by the PD 10 is also sent toa circuit (not illustrated) for controlling an output of a semiconductorlaser and also sent to the arithmetic circuit 11.

The arithmetic circuit 11 stores a value a of the ratio between p and qwhen the distance between the SIL 2 and the optical disk 3 is equal to acertain distance m at which the reflectance of the high-frequencycomponent at the facing plane of the SIL 2 is small enough (i,e, thehigh-frequency component is sufficiently irradiated to the optical disk3) as a control target value.

The arithmetic circuit 11 computes the ratio x between the light amountsp and q. When the sign of the difference x-a between the x and thecontrol target value a is positive (i,e, when the distance between theSIL 2 and optical disk 3 is larger than m), the circuit generates asignal for controlling the movement of a focus actuator (notillustrated) so as to make the objective lens 1 and SIL 2 move in adirection for approaching the optical disk 3 by a distance correspondingto the difference magnitude. On the other hand, when the sign of x-a isnegative (i,e, when the distance between the SIL 2 and optical disk 3 issmaller than m), the circuit generates a signal for controlling themovement of the focus actuator so as to make the objective lens 1 andSIL 2 move in a direction for going away from the optical disk 3 by adistance corresponding to the difference magnitude and sends the controlsignal fs to the focus actuator.

The focus control operation by the optical disk drive is performed asfollows.

When a laser beam is emitted from a semiconductor laser, the partiallight amount q of the laser beam is detected by the PD 10, and the lightamount p of the reflected light of the high-frequency component at thefacing plane of the SIL 2 is detected by the PD 8.

When the distance between the SIL 2 and optical disk 3 is larger thanthe above value m, the sign of the difference x-a is positive becausethe light amount of the reflected light of the high-frequency componentat the facing plane of the SIL 2 increases and so, the ratio x becomeslarger than the control target value a.

Therefore, at this moment, the objective lens 1 and SIL 2 are moved bythe focus actuator in a direction of approaching the optical disk 3 by adistance corresponding to the value of the difference x-a on the basisof the control signal from the arithmetic circuit 11.

However, when the distance between the SIL 2 and optical disk 3 issmaller than m, the light amount of the reflected light of thehigh-frequency component at the facing plane of the SIL 2 decreases, sothat the ratio x becomes smaller than the control target value a, andthus the sign of the difference x-a turns negative.

Therefore, at this moment, the objective lens 1 and SIL 2 are moved bythe focus actuator in a direction of going away from the optical disk 3by a distance corresponding to the value of the difference x-a.

In this way, focus control is performed so that the ratio x may convergeon the control target value a (i,e, the distance between the SIL 2 andoptical disk 3 may converge on m).

In this case, because a change of the light amount of the reflectedlight of the high-frequency component at the facing plane of the SIL 2is large enough even in the near field range, it is possible to performprecise focus control by accurately detecting a change of the distancebetween the SIL 2 and optical disk 3 from a detection result by the PD8.

Moreover, even if the intensity of a laser beam emitted from asemiconductor laser changes, p and q change at the same rate, so thatthe ratio x does not change. This makes it possible to detect a changeof the distance between the SIL 2 and optical disk 3 regardless of theintensity of the laser beam.

Furthermore, a shape of the SIL 2 does not change at all from aconventional one and besides, the monitoring PD 10 for controlling anoutput of a semiconductor laser is used as it is to find the ratio x.Therefore, the number of new devices to be added is minimized.

Also, reflection of a laser beam incoming to the SIL 2 at an incidenceangle of θ₀ or less at the facing plane of the SIL 2 and reflection of alaser beam at a portion of the optical disk 3 closer to the SIL 2 than asignal recording plane of the disk 3 are suppressed by theantireflection film 4, so that interference between the reflected lightof the high-frequency component and other light at the facing plane ofthe SIL 2 decreases. Thus, deterioration of an accuracy for detecting achange of the distance between the SIL 2 and optical disk 3 due to theabove interference is prevented. This point also makes it possible tomore accurately detect a change of the distance.

Additionally, in the above example, the present invention is applied toan optical disk drive having an optical system in which an SIL is setbetween an objective lens and an optical disk.

However, the present invention may be applied not only to the aboveoptical disk drive but also to an optical disk drive having an opticalsystem realizing functions of an objective lens and an SIL by a singleoptical device, an optical system realizing functions of an objectivelens and an SIL by three or more optical devices, and an optical systemrealizing functions of an objective lens and an SIL by a hologramdevice.

Moreover, the above example has been described assuming that the presentinvention is applied to an optical disk drive. However, the presentinvention is also applicable to an exposure device for a master opticaldisk or to a recording/reproducing apparatus of an optical recordingmedium other than an optical disk or an exposure device for a masterdisk of that optical recording medium.

Next, an example of a beam irradiator will be described, which isapplied to an optical unit serving as a pickup unit of an optical diskdrive, or an exposure device for a master disk of an informationrecording medium such as an optical recording medium (e.g. optical disk)and others.

Also, a focus control method, a totally-reflected-light detecting methodand the like to be applied to the beam irradiator will be described.

In an optical recording medium (e.g. optical disk), it is necessary todrive a convergent lens used for recording or reproducing informationand perform focus control so that an information recording layer mayalways fall within the focal depth of the convergent lens. The sameapplies to a case of exposing a master optical disk.

As a wavelength of light used for an optical recording medium hasrecently been shortened, a focal depth of a spot has been shallowed evenif a numerical aperture NA of a convergent lens is the same andtherefore, more-stable focus control has been essential.

Thus, the present invention intends to perform low-frequency-band focuscontrol utilizing an air suspension according to simultaneous use ofpositive-pressure air and negative-pressure air and also to addhigh-frequency-band focus control by shift means using an electricaldriving element for performing more-stable focus servo.

FIG. 3 shows an example of a schematic structure of a focus controlmechanism of a convergent lens of a beam irradiator using the presentinvention, which is adapted for these purposes.

The beam irradiator is provided with a beam generating source forgenerating at least any one of a light beam, an electron beam and an ionbeam as well as a convergent lens for converging a beam supplied fromthe beam generating source. In FIG. 3, a numeral 21 denotes a convergentlens and 22 denotes an irradiated object.

The focus control mechanism of this example is able to perform the focuscontrol of the convergent lens 21 by combining a first control mechanism24 for performing focus control in a low-frequency band (for so-calledlow-frequency swell of the irradiated object 22) by positive-pressureair (high-pressure air) and negative-pressure air with a second controlmechanism 25 for performing focus control in a high-frequency band (forso-called high-frequency swell of the irradiated object 22) using, e,g,a piezostack forming the shift means 26.

The convergent lens 21 is fixed to an actuator 27 whose position isadjusted relative to the irradiated object 22 such as a disk set on arotational base. The actuator 27 is provided with the first and secondfocus control mechanisms 24 and 25. The convergent lens 21 has anobjective lens (aspherical lens) 31 and an SIL 32 held in a lens tube 28on the same optical axis. The SIL 32 of this example has a shapeobtained by cutting off a part of a spherical lens to leave more than ahemisphere and has a columnar protrusion 32 a formed at the bottomthereof.

The actuator 27 is formed by fixing a cylindrical air pad 35 to befloated from the irradiated object 22 by intervention of an air layer toan arm (so-called lens fixing setting jig) 33 through an elastic body 34such as a flat spring. The shift means 26 using a cylindrical piezostackis fixed to the upper end in the air pad 35. The lens tube 28 holdingthe convergent lens 21 is supported in the cylindrical air pad 35through the shift means 26, and the bottom end face of the SIL 32 (i,e,end face of the columnar protrusion 32 a) is set to a plane of the airpad 35 facing the irradiated object 22.

An air jet port (air supply port) for jetting air, a so-calledpositive-air jet port 36 and an air intake port for taking in air, aso-called negative-pressure intake port 37 are provided on a plane ofthe air pad 35 facing the irradiated object 22.

As shown in FIG. 4 for example, the air jet port 36 and air intake port37 are formed annularly and concentrically to the central axis of theair pad 35. The air jet port 36 can be made of a porous member such asporous carbon. The porous carbon is fitted to an annular groove formedon the face of the air pad 35.

High-pressure air is supplied from a high-pressure air supply source 39to the air jet port 36 through a pipe 38. Air is taken into an airintake means 41 from the air intake port 37 through a pipe 40.

A high-pressure-air supply source 39 is provided with as control means42 for controlling, for example, supply quantity and pressure of the airand the air intake means 41 is provided with a control means 43 forcontrolling, for example, intake quantity and pressure of the air. Thepositive-pressure air and negative-pressure air perform initial roughadjustment for making the bottom end face of the convergent lens 21 facethe irradiated object 22 while keeping a necessary distance. In thismanner, the first focus control mechanism 24 for selecting a position ofthe convergent lens 21 is constructed.

Moreover, a voltage is supplied to, for example, the piezostack formingthe shift means 26 from a voltage supply section 45 so as to minutelymove the convergent lens 21 a long the axis of the air pad 35 inaccordance with the displacement due to the piezoelectric effect of thepiezostack. In this way, the second focus control mechanism 25 formaking position control of the convergent lens 21, i,e, focus controlrelative to the irradiated object 22 is constructed.

The voltage supply section 45 supplies a voltage corresponding to afocus error to, for example, the piezostack forming the shift means 26in accordance with a focus servo signal supplied from a detectionsection 46 for detecting a focus error.

In addition, it is possible to make the air jet port 36 divided into aplurality of ports, e,g, three ports in this example as shown in FIG. 5,so as to supply high-pressure air to air jet ports 36 a, 36 b, and 36 cthrough pipes 38 a, 38 b, and 38 c, respectively. as shown in FIG. 5. Inthis case, it is also possible to provide high-pressure-air supplysource and control means for then respective pipes 38 a, 38 b, and 38 cso that air pressure and air supply quantity of each port can becontrolled. Also, it is possible to divide the air intake port 37 into aplurality of ports so as to take in air from each port.

Furthermore, though not illustrated, it is possible to make the air jetport 36 and air intake port 37 of a multiplicity of openings along acircle instead of limiting them to the annular shape.

The focus control mechanism comprised of the first and second focuscontrol mechanisms 24 and 25 float the air pad 35 above the irradiatedobject 22 (for example, a base plate 48 whose surface is coated with aphotoresist layer 47) using a pressure of positive-pressure air.Specifically, the air pad 35 is floated above the irradiated object 22by jetting positive-pressure air from the bottom plane of the air pad 35through the air jet port 36 and attracted toward the irradiated object22 by negative-pressure air taken in through the air intake port 37. Bysetting a pressure of positive-pressure air at, e,g, 5 kgf and apressure of negative-pressure air at, e,g, a value equal to atmosphericpressure—100 mmHg, it is possible to float the air pad by, e,g, 5 μm.

Though the air pressure is fixed in this case, rigidity is produced inan air film formed between the irradiated object 22 and the air pad 35due to a balance of the positive and negative pressure of air. When aswell of the surface of the irradiated object 22 is associated with alow-frequency band, the air pad 35 can follow the swell. Therefore, aresponse speed of the air pad 35 shifting in accordance with a verticaldisplacement of the irradiated object 22 will be constant for bothupward and downward displacements.

On the other hand, by applying a voltage to, e,g, the piezostack formingthe shift means 26 to extend or contract the piezostack, it is possibleto apply servo corresponding to a high-frequency-band swell which cannotbe followed by the air pad 35 only as described above and performhigh-frequency-band position control, namely, focus control.

Because the air pad 35 is fixed to the arm 33 through the elastic body34 using a dogleg-shaped or U-shaped flat spring, it is possible to givefreedom from not only a vertical direction but also a tilt to the airpad 35.

According to a beam irradiator provided with the above focus controlmechanism, stable focus control in a low-frequency band is enabled bysimultaneously using positive-pressure air 49 and negative-pressure air50. At the same time, a constant response speed is also obtained.Because the focus control mechanism is separated into a high-frequencyband and a low-frequency band, the improvement of a servocharacteristics in each band is expected. Therefore, it is possible tokeep the distance between the convergent lens 21 and the irradiatedobject 22 accurately and stably.

In the present invention, the convergent lens 21 is comprised of twolens groupes including the SIL 32 and the objective lens 31. However,the convergent lens 21 may be made of any lens which converges a lightbeam (e.g. laser beam),e,g, objective lens formed by an SIM (solidimmersion mirror) 51 shown in FIG. 6, a lens group including an SIM, orother objective lens formed by three groups or more having been used sofar. In this case also, it is possible to stably perform servo with thesame accuracy as the above.

As concerns the SIM 51, an light-beam-L incoming side has a concaveface, but a light-beam-L incident portion has a convex face as shown inFIG. 6. A light-beam-L emitting side has a flat face. In this case, theflat face is a plane in which the columnar protrusion 52 is provided atthe center thereof as described above. A reflection film 54 is formed ona convex face of the transparent body 53 formed and the flat face of thebody 53 excluding the columnar protrusion 52.

The light beam L enters the concave face and is refracted. It is thenreflected from the reflection film 54 at the flat face, furtherreflected from the reflection film 54 on the upper convex face, andconverged on the end face of the columnar protrusion 52.

Although the present invention uses a piezostack being as an electricaldriving element as the shift means 26 for focus control in ahigh-frequency band, other electrical driving element such as anelectromagnetic coil, so-called voice coil may be used.

While pressures of positive-pressure air and negative-pressure air arefixed in the above example, the air pressures may be controlled. Whenthe air pressures is fixed, the air pad 35 vertically follows inaccordance with a vertical displacement of the irradiated object 22. Inorder, to further accelerate the follow, it is also possible to increasethe response speed of the air pad 35 by increasing the pressure ofnegative-pressure air when the irradiated object 22 shifts downward andincreasing the pressure of positive-pressure air when the object shiftsupward.

In the above example, focus servo in a lower-frequency band by air andfocus servo in a high-frequency band by the piezostack of the shiftmeans 26 are used at the same time. However, when a high-frequency-bandswell of a face on which light is condensed by the convergent lens 21 issmall, it is allowed to use only servo by air.

When oscillating a 350-nm laser beam used mainly at the recent exposingstep of a master optical disk by an SHG (Secondary Harmonic Generator),its wavelength is approx. 170 nm. The sensitivity of photoresist becomeseffective at a wavelength of approx. 450 nm or less. In view of thesefacts, a beam irradiator using the present invention is preferablyprovided with a generator for producing a laser beam having a wavelengthbetween 170 and 450 nm inclusive.

The so-called attract-flat-type air pad 35 using positive-pressure airand negative-pressure air shown in FIG. 3 well follows a swell on thesurface of the object 22 to be irradiated.

However, if the air jet port 36 and air intake port 37 of theattract-float-type air pad 35 are improperly arranged, for example, ifthe air intake port 37 is concentrically set outside the annular air jetport 36 set on the bottom of the air pad 35 the air pad 35 tilted due tovery small vibration is further tilted because of the air intake port 37present on the outer periphery and the edge of the air pad 35 maycontact with the object 22 to be irradiated. Therefore, it is necessaryto carefully arrange these air jet port 36 and air intake port 37.

Moreover, when the air pad 35 is floated, only the air jet port 36descends onto the object 22 to be irradiated. In this case, if the tipend of the convergent lens 21 protrudes beyond the bottom of the air pad35, the convergent lend 21 may collide with the object 22 to beirradiated. Therefore, it is necessary to take countermeasure foravoiding collision.

Furthermore, the outer periphery of the front end of the SIL 32 set onthe air pad 35 is scraped through etching while leaving the columnarprotrusion 32 a having, e,g, a diameter of 40 μm and a height of 2 μm.This reduces, the probability for the lens, i,e, the SIL 32 to contactwith the irradiated object 22 when the lens tilts. Still, to decreasethe distance (so-call gap) between the convergent lens 21 and theirradiated object 22 down to, e,g, approx. 40 nm, it is necessary topreviously adjust a tilt of the convergent lens 21 with an accuracy of 1mrad or less. A method and an apparatus for facilitating the aboveadjustment with high accuracy is desired.

Thus, a beam irradiator concerning the present invention is constructedso that an air jet port provided for the lowermost portion, i,e, thebottom of an attract-float-type air pad using air pressure may bearranged outside an air intake port.

Moreover, the beam irradiator is constructed so as to retreat, in theinitial state the lowermost face (e.g. bottom of an SIL or SIM) of aconvergent lens up to a position higher than the height from the surfaceof an irradiated object to the bottom of an air pad and then lower thelowermost face of the convergent lens down to a position lower than theheight from the surface of the irradiated object to the bottom of theair pad during beam irradiation (e.g. when exposing a master disk orrecording or when reproducing data in or from an information recordingmedium).

A lens-tilt adjuster used for the present invention is arranged so as tohave a mechanism for adjusting a tilt when fixing an convergent lens bya screwing pressure through an elastic member inserted between contactfaces for fixing the convergent lens to shift means and/or contact facesfor fixing the shift means provided with the convergent lens to an airpad.

A lens-tilt adjusting method used for the present invention sets aconvergent lens on a substrate having a high reflection surface andadjusts a tilt of a convergent lens, so-called the optical axis of thelens by conforming the axis of returned light reflected from the bottomof the convergent lens (e.g. convergent lens including SIL or SIM) withthe axis of returned light which is reflected from the high reflectionsurface of the substrate after permeating the convergent lens andpermeates again the same lens.

A method using returned light totally reflected from the bottom plane ofthe SIL 32 (bottom of the columnar protrusion 32 a) is one of the signaldetection methods when controlling the distance between the convergentlens 21 and the irradiated object 22 by making the distance close to anear field (close-field area). In this case, because control isperformed at a position where the intensity of totally-reflectedreturned light is small, it is necessary to detect the totally-reflectedreturned light at larger intensity and also decrease noises.

A totally-reflected-light detecting method according to the presentinvention separates most of returned-light power in a directiondifferent from that of an incoming-beam source by convertinglinearly-polarized laser beam emitted through a polarized-beam splitterinto circularly-polarized rays by a ¼-wavelength plate makingthereafter, the circularly-polarized enter a convergent lens, changingthe polarization direction of the light returned from the convergentlens by the ¼-wavelength plate, and making the returned light enter thepolarized-beam splitter again.

Furthermore, the totally-reflected-light detecting method mainly passthe light totally reflected from the bottom face of a convergent lens(SIL or SIM lens) by arranging a circular mask on an optical path of thelight returned from the convergent lens, which is adjusted to theoptical axis of the convergent lens, and has a circular transmissionarea or has a transmission area on the outer periphery, and interruptingthe light interfering on a plurality of faces including the bottom faceof the convergent lens.

FIG. 7 shows a schematic structure of another example of focus controlmechanism of a convergent lens in a beam irradiator thus improved by thepresent invention.

Also, FIGS. 8 to 11 show an example of performing optical-axisadjustment and focus control of a convergent lens using the lens-tiltadjusting method and the totally-reflected-light detecting method.

In this example, as shown in FIG. 7, a cylindrical air pad 35 having ajet port 36 of positive-pressure air 49 and an intake port 37 ofnegative-pressure air 50 on the facing plane facing an irradiated object22 is fixed to an arm 33 through an elastic body 34 such as a flatspring similarly to the case of FIG. 3, and a lens tube 28 holding aconvergent lens 21 is supported inside the air pad 35 through an annularpiezostack serving as shift means 26 so as to shift in the axisdirection.

The air jet port 36 and air intake port 37 are each formed annularly,i,e, concentrically around the axis of the air pad 35, and particularlythe air jet port 36 is provided so as to be located on the outerperiphery of the air intake port 37.

The convergent lens 21 is constructed so as to hold an objective lens 31and an SIL 32 on the same optical axis similarly to the case describedabove. The convergent lens 21 is set so that the bottom of the SIL 32may retreat from the bottom plane of the air pad 35 facing theirradiated object 22 in the initial state, i,e, when no voltage isapplied to the piezostack of the shift means 26, but can protrude beyondthe bottom plane of the air pad 35 and extend by a float amount or overwhen a voltage is applied to the piezostack of the shift means 26.

An elastic member elastic in the thickness direction, preferably anelastic sheet such as a gel-type sheet 57 is set on the face fixing thepiezostack being the shift means 26 to the air pad 35 so as to make thepiezostack and the air pad 35 into one body by a screw 56 at a pluralityof positions, preferably at three or more positions, for example, fourpositions as shown in FIG.4. A numeral 58 denotes a base plate connectedto the piezostack and the piezostack is screwed to the base plate 58.

Because other structures are the same as those described with FIG. 3,corresponding parts are denoted by the same numerals and its repeatedexplanation will be omitted.

A case will be described where a beam irradiator provided with a focuscontroller including the convergent lens 21 shown in FIG. 7 isapplied,e,g, to an exposure device for manufacturing a master disk toobtain a stamper for manufacturing an information recording medium.

FIGS. 8 and 9 show an optical system of the beam irradiator.

The optical system is provided with a beam splitter (e.g. half mirror)61, a polarized-beam splitter (PBS) 62, a ¼-wavelength plate (QWP) 63,and a mirror 64 on the optical path of a laser beam L emitted from alaser-beam generator (not illustrated) so that converted into parallelrays by a collimator lens (not illustrated) and a laser beam reflectedfrom the mirror 64 may enter a convergent lens 21. Moreover, a screen65, an opaque mask 66, a condenser lens 67, and a first detection devicesuch as a photodetector 68 are arranged on an optical path which isconverted by the polarized-beam splitter 62 converting an optical pathof the returned light including the light reflected from the SIL 32 ofthe convergent lens and the light reflected from the irradiated object22. The screen 65 and opaque mask 66 are arranged to be movable betweena position on the optical path and a position off from the optical path.Further, a second detection device such as a photodetector 70 is set onan optical path of a laser beam reflected from the beam splitter 61through a condenser lens 69.

First of all, a tilt, i,e, an optical axis of the convergent lens 21 isadjusted.

In the lens-tilt adjusting method, as shown in FIG. 8, a substrate 72 isprepared whose surface is made into a high-reflection plane 71 throughmetal plating or the like and an air pad 35 provided with the convergentlens 21 is placed on the high-reflection plane 71 of the substrate 72.Under this state, the axis of the air pad 35 is perpendicular to thesubstrate 72.

A laser beam L emitted from a laser beam generator as linearly-polarizedrays and converted into circularly-polarized rays through the beamsplitter 61, the polarized-beam splitter 62 and the ¼-wavelength plate63 is aligned perpendicular to the face of the substrate 72 through themirror 64 and made to enter the convergent lens 21 in the air pad 35.The returned light reflected from the convergent lens 21, i,e, thebottom plane of the columnar protrusion 32 a of the SIL 32 and thesubstrate 72 is reflected from the mirror 64 and permeates the¼-wavelength plate 63, thereafter being separated by the polarized-beamsplitter 62 for changing its optical path and projected to the screen65.

The light most of which is totally reflected from the bottom plane ofthe SIL 32 of the convergent lens 21 returns as an orbicular (so-calledringlike) beam as described below. The beam reflected from the surface71 of the substrate 72 is scattered by the columnar protrusion 32 a ofthe SIL 32 and its projected image is reflected again from the substrate72 and returns as a convergent beam. The convergent beam returns asinterference light produced between the SIL 32 and the substrate 72.

Thus, as shown in FIG. 10, the orbicular beam 74 totally reflected fromthe SIL 32 and interference light (interference fringe) 75 producedbetween the columnar protrusion 32 a at the center of the SIL 32 and thesubstrate 72 are projected to the screen 65, thereby enabling an offsetbetween their optical axes O₁ and O₂ to be confirmed.

Assuming that the center of the columnar protrusion 32 a of the SIL 32is aligned with the optical axis, fastening degrees of four screws 56for fixing the piezostack of the shift means 26 to the air pad 35through the gel-type sheet 57 are adjusted so that the optical axis ofthe orbicular beam 74 may be aligned with that of the interference light75. This makes the optical axis of the convergent lens 21 becomesvertical to both the substrate 72 and the bottom of the air pad 35 andalso the bottom plane of the SIL 32 parallel with the substrate 72.

In this example, the gel-type sheet 57 is inserted between contact facesof the air pad 35 and the piezostack of the shift means 26.Alternatively or in addition thereto, the gel-sheet 57 may be insertedbetween contact faces of the piezostack as the shift means 26 and thelens tube 28 for holding the convergent lens 21.

In this context, as shown in FIG. 12, when a refractive index n and amaximum incidence angle θ_(max) are set so that a numerical aperture NAmay be equal to a predetermined value, e,g, so that a product n·sinθ₀concerning a certain incidence angle θ₀ smaller than the maximumincidence angle θ_(max) and the refractive index n may be equal to 1.0as described above, the light entering the SIL 32 from the objectivelens 21 at an incidence angle at which the numerical aperture becomeslarger than the predetermined value, that is, 1 (high-frequencycomponent of incoming light) almost permeates a plane of the SIL 32facing a substrate when the SIL 32 contacts with the substrate and isirradiated to the substrate if the SIL 32 contacts with the substrate.However, as the SIL 32 moves away from the substrate, the reflectance ofthe high-frequency component at the facing plane suddenly increases.When the SIL 32 moves away from the substrate beyond the near field,almost 100% of the light is reflected from the facing plane. Therefore,the returned light reflected from the bottom plane of the SIL 32 formsan orbicular shape (see the hatched portion in FIG. 12) and is projectedonto the screen 65 as sown in FIG. 10. Moreover, the light incoming tothe SIL 32 from the objective lens 31 at an incidence angle at which thenumerical aperture is smaller than the predetermined value, or 1.0(low-frequency component of incoming light) passes through the SIL 32and is reflected from the surface of the substrate 72 and projected ontothe screen 65 as the interference light 75.

Next, master-disk exposure is started.

The positive-pressure air 49 is jetted (to supply air) from the bottomof the air pad 35 through the air jet port 36, i,e, annularly-arrangedporous carbon to float the air pad 35. Then, as shown in FIG. 9, anirradiated object, namely, a master disk 22 having the photoresist layer47 applied to the surface of the substrate 48 is prepared and the airpad 35 is lowered onto the master disk 22 while keeping the master disk22 stationary and the air pad 35 horizontal.

When the air pad 35 is lowered until a support load of the air pad 35becomes 0 by floating, intake by the negative-pressure air 50 (airintake) is performed through the air intake port 37. For example,suppose a supply-air pressure is set at 5 kgf and an intake-air pressureis set at a value equal to atmospheric pressure—100 mmHg. If theintake-air pressure is too high, the air pad 35 may collide with themaster disk 22 due to the action of a down-force. Inversely, if theintake-air pressure is too low, the air pad 35 cannot follow a swell onthe master disk, if any and will be repelled due to a floating force bypositive pressure. Rigidity is produced in an air film depending on thebalance between the positive- and negative-air pressures as describedwith FIG. 3 and so the air pad 35 follows the swell of the master disk22.

Then, the exposure step is entered. At this moment, as shown in FIG. 9,the screen 65 retreats to a position out of an optical path and theopaque mask 66 is set into the optical path instead. As shown in FIG.11, the opaque mask 66 is formed in such a size as interrupts thereturned light of a so-called low-frequency component.

During exposure, a predetermined voltage is applied to the piezostack ofthe shift means 26 and the convergent lens 21 protrudes from the bottomplane of the air pad 35 so that the distance between the SIL 32 of theconvergent lens 21 and the master disk 22 may fall within the nearfield.

A linearly-polarized laser beam emitted from a laser beam generator (notillustrated) is converted into parallel rays by a collimator lens (notillustrated) and enters the polarized-beam splitter 62 after passingthrough the beam splitter 61. The laser beam L passing through thepolarized-beam splitter 62 and converted into circularly-polarized raysby the ¼-wavelength plate 63 is reflected from the mirror 64 toselectively make the photoresist layer 47 on the master disk 22 exposedthrough the convergent lens 21.

Focus control is performed during the above exposure step.

An exposure beam, i,e, the laser beam L is made to enter the convergentlens 21, a laser beam incoming to the SIL 32 from the objective lens 31at an incidence angle larger than the above θ₀ (high frequencycomponent) being reflected from the bottom plane of the SIL 32, thereflected laser beam being converted through the objective lens 31 andthe mirror 64 into linearly-polarized rays orthogonal to the originalrays by the ¼-wavelength plate 63 and reflected from the polarized-beamsplitter 62, and the light intensity of the rays being detected by thephotodetector 68 for monitoring the beam intensity through the condenserlens 67 as a voltage value.

On the other hand, a part of the laser beam emitted from the laser beamgenerator is reflected from the beam splitter 61 and the light intensityof the partial laser beam is detected by the photodetector 70 formonitoring the beam intensity as a voltage value through the condenserlens 69.

In this example, the totally-reflected light amount of a laser beam(high-frequency component) when sufficiently moving the SIL 32 away fromthe master disk 22 is defined as a reference light amount. Therefore, avalue obtained by multiplying a value derived from the photodetector 70by a predetermined coefficient is sent to an arithmetic circuit 76 as asignal corresponding to the reference light amount. A signal showing alight amount detected by the photodetector 68 is also sent to thearithmetic circuit 76.

The arithmetic circuit 76 stores a constant-value level when thedistance between the SIL 32 and the master disk 22 is equal to a certaindistance, namely, a distance falling within the near field, for example,a level falling down to 60% of the reference light amount as a targetcontrol value.

The arithmetic circuit 76 generates a focus-control signal in accordancewith the difference between the target control value and a light amountsupplied from the photodetector 68. The control signal is sent to thefocus-error detection section 46 to control the telescopic motion of thepiezostack of the shift means 26 so as to keep the light amount suppliedfrom the photodetector 68 when the light amount reaches theconstant-value level relative to the reference light amount. This makes,it possible to keep the distance (gap) between the SIL 32 and the masterdisk 22 constant.

This state, namely, a state in which the totally-reflected light amountdecreases is produced when the SIL 32 approaches the master disk 22within the near field. It can be seen from this fact that if thedistance (gap length) can be kept stably, then the distance can becontrolled within the near field.

When the distance between the SIL 32 and the master disk 22 becomesequal to or less than a wavelength of a laser beam by controlling thelength of the piezostack of the shift means 26, a laser beam of ahigh-frequency component incoming to the SIL 32 penetrates toward themaster disk 22 and thus, a reflected light amount decreases. At thismoment, an interference fringe produced between the bottom plane of theSIL 32 and the master disk 22 is superposed on the orbiculartotally-reflected returned light due to total reflection and intensityvibrations due to the interference fringe form noises in the distance(gap length) control according to totally-reflected light.

As shown in FIG. 11, however, the circular opaque mask 66 through whichthe orbicular totally-reflected light (hatched portion) passes isarranged on the optical path of the light LR returning to thephotodetector 68, whereby interference light is eliminated and onlytotally-reflected returned light mainly passes. The opaque mask 66 makesit possible to control intensity vibrations to a degree of not affectingthe control of the distance (gap length) between the SIL 32 and themaster disk 22.

As the result of feeding back a change of totally-reflected lightintensities to a voltage applied to the piezostack of the shift means 26and servo-controlling the above distance (gap length) to a position ofapprox. 100 nm while keeping the master disk 22 stationary, fluctuationof the distance (gap length) could be kept within a range of approx. 1nm at the maximum. Moreover, as the result of rotating the master disk22 at approx. 600 rpm and performing distance (gap length) control at aposition of a radius of approx. 40 mm, the magnitude of the fluctuationcould be controlled to approx. 10 nm at the maximum.

As described above, in this example, because of arranging thepositive-pressure-air jet port 36 on the outer periphery side of thenegative-pressure-air intake port 37, positional balance between jet andintake of air is kept, so that it is possible to float the air pad 35 ina stable attitude.

By retreating the SIL 32 from the bottom plane of the air pad 35, it ispossible to avoid collision of the SIL 32 with a master disk except whencontrolling a near-field gap length.

Because the air pad 35 and shift means 26 are fixed to each other byscrews through an elastic member such as the gel-type sheet 57, it ispossible to easily perform tilt adjustment, i,e, so-called optical-axisadjustment of the convergent lens 21 with high accuracy using theflexibility of the gel-type sheet 57.

Moreover, it is possible to realize high-accuracy adjustment by a visualeasy method in tilt adjustment of the SIL 32 for which a high-accuracyparallelism with the irradiated object 22 is required.

Furthermore, it is possible to increase the intensity oftotally-reflected returned light used for the above distance (gaplength) control by using the polarized-beam splitter 62 and ¼-wavelengthplate 63, control the intensity of interference light forming noises byusing the opaque mask 66, and increase a ratio ofdetection-signal-intensity-to-noise-intensity (S/N ratio) in thedistance(gap length) control, which makes the control to be performedwith high accuracy.

INDUSTRIAL APPLICABILITY

The above described beam irradiator is not restricted to those used forexposure of the photoresist layer 47. For example, it is possible toform into an optical apparatus serving as an optical recorder forrecording information in the irradiated object 22 such as a recordableinformation recording medium, e,g, the so-called CD-R, or amagneto-optical recording medium having a magneto-optical recordinglayer, or an information recording medium having a recording layer inwhich data is recorded in accordance with a phase change, or into anoptical apparatus serving as the so-called pickup for reproducing therecorded information.

Moreover, the present invention makes it possible to construct arecording and/or reproducing apparatus for recording and/or reproducinginformation by comprising an optical apparatus having of the aboveconstructed beam irradiator.

A beam irradiator, focus control method, totally-reflected-lightdetecting method, and distance-change detecting method relating to thepresent invention can be applied to purposes other than the aboveexamples.

It is a matter of course that the present invention is not restricted tothe above examples and it is able to have any one of other variousstructures without departing from the gist of the present invention.

1. A distance-change detecting method for detecting a change of thedistance between a second optical means and an optical recording mediumin an optical system having a first optical means for condensing thelight to irradiate the optical recording medium and the second opticalmeans being set between the first optical means and the opticalrecording medium in order to realize a numerical aperture larger thanthat of the first optical means, the method comprising the steps of:detecting the light incident to the second optical means from the firstoptical means at an incidence angle at which a numerical aperture islarger than a predetermined value and reflected from a plane facing theoptical recording medium out of the second optical means; detecting thedistance change in accordance with the amount of the reflected light;calculating a light amount ratio between a high frequency component ofthe incident light and a high frequency component of the reflectedlight; and controlling a distance between the optical recording mediumand the second optical means so that the distance between the disk andthe second optical means is constant in a near-field range, wherein oneof the light incoming to the first optical means and the light incomingto the second optical means is detected, and wherein the distance iscontrolled based on the calculated ratio.
 2. The distance-changedetecting method according to claim 1, wherein the second optical meansis a solid immersion lens, and the light incoming to the second opticalmeans from the first optical means at an incidence angle at which anumerical aperture is larger than 1 and reflected from a plane of thesecond optical means facing the optical recording medium is detected. 3.A distance-change detector for detecting a change of the distancebetween a second optical means and an optical recording medium in anoptical system having a first optical means for condensing the light toirradiate the optical recording medium and the second optical means setbetween the first optical means and the optical recording medium inorder to realize a numerical aperture larger than that of the firstoptical means, the detector comprising: a light detection means fordetecting the light incoming to the second optical means from the firstoptical means at an incidence angle at which a numerical aperture islarger than a predetermined value and reflected from a plane of thesecond optical means facing the optical recording medium; a firstdetection means for controlling the distance between the opticalrecording medium and the second optical means so that the-distancebetween the optical recording medium and the second optical means isconstant in a near-field range; and a second light-detection means fordetecting one of the light incoming to the first optical means and thelight incoming to the second optical means, wherein the detection meansis comprised of comparison means for calculating a ratio between a highfrequency component of the incident light and a high frequency componentof the reflected light.
 4. The distance-change detector according toclaim 3, wherein the second optical means is a solid immersion lens, andthe light detection means detects the light incoming to the secondoptical means from the first optical means at an incidence angle atwhich a numerical aperture is larger than 1 and reflected from a planeof the second optical means facing the optical recording medium.
 5. Thedistance-change detector according to claim 4, wherein a member forcontrolling reflection of light is formed on at least one of a plane ofthe second optical means facing the optical recording medium and aportion of the optical recording medium closer to the second opticalmeans than a signal recording plane of the optical recording medium. 6.The distance-change detector according to claim 3, wherein a member forcontrolling reflection of light is formed on at least one of a plane ofthe second optical means facing the optical recording medium and aportion of the optical recording medium closer to the second opticalmeans than a signal recording plane of the optical recording medium. 7.The distance-change detector according to claims 3, wherein a member forcontrolling reflection of light is formed on at least one of a plane ofthe second optical means facing the optical recording medium and aportion of the optical recording medium closer to the second opticalmeans than a signal recording plane of the optical recording medium. 8.A focus control method for controlling the distance between a secondoptical means and a optical recording medium in an optical system havinga first optical means for condensing the light to irradiate the opticalrecording medium and a second optical means set between the firstoptical means and the optical recording medium in order to realize anumerical aperture larger than that of the first optical means, themethod comprising the steps of: detecting the light incident to thesecond optical means from the first optical means at an incidence angleat which a numerical aperture is larger than a predetermined value andreflected from a plane out of the second optical means facing theoptical recording medium; calculating a light amount ratio between ahigh frequency component of the incident light and a high frequencycomponent of the reflected light; and controlling a distance between theoptical recording medium and the second optical means so that thedistance between the optical recording medium and the second opticalmeans is constant in a near-field range, wherein the distance iscontrolled based on the calculated light amount ratio.
 9. The focuscontrol method for controlling the focus of a convergent lens accordingto claim 8, further comprising the steps of: floating the convergentlens above the face of an irradiated object by positive-pressure air;and attracting the convergent lens to the irradiated object bynegative-pressure air.
 10. The focus control method according to claim9, wherein the distance between the convergent lens and the irradiatedobject is controlled by controlling the air pressures.
 11. The focuscontrol method according to claim 9 or 10, wherein a focal position iscorrected by driving the convergent lens with an electrical drivingmeans.
 12. A focus controller for controlling the distance between asecond optical means and an optical recording medium in an opticalsystem having a first optical means for condensing the light toirradiate the optical recording medium and the second optical means setbetween the first optical means and the optical recording medium inorder to realize a numerical aperture larger than that of the firstoptical means, the controller comprising: a light detection means fordetecting light incident to the second optical means from the firstoptical means at an incidence angle at which. a numerical aperture islarger than a predetermined value and reflected from a plane of thesecond optical means facing the optical recording medium; a detectionmeans for calculating a light amount ratio between a high frequencycomponent of the incident light and a high frequency component of thereflected light and detecting a distance change in accordance with thecalculated ratio; and a control means for controlling the distancebetween the optical recording medium and the second optical means basedon the calculated ratio so that the distance between the opticalrecording medium and the second optical means is constant in anear-field range.